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Abstract:

A pair of coaxial electrodes 10 that face each other, a
discharge-environment-maintaining device 20, and a voltage-applying
device 30 are provided. Each coaxial electrode 10 includes a center
electrode 12, a guide electrode 14 which surrounds the front end portion
of the facing center electrode, and an insulation member 16 which
insulates the center electrode and the guide electrode from each other.
The insulation member 16 is formed of partially porous ceramics including
an insulative dense portion 16a and a porous portion 16b. The insulative
dense portion 16a includes a reservoir 18 which holds a plasma medium
therein, and by the porous portion 16b, the inner surface of the
reservoir 18 communicates with a gap between the center electrode 12 and
the guide electrode 14 through the inside of the insulative dense portion
16a.

Claims:

1. A plasma light source comprising: a pair of coaxial electrodes that
face each other; a discharge-environment-maintaining device that
maintains a plasma medium inside the coaxial electrodes at a temperature
and a pressure appropriate for the generation of plasma; and a
voltage-applying device that applies discharge voltages having reversed
polarities to the respective coaxial electrodes, wherein a tubular
discharge is formed between the pair of coaxial electrodes so as to
confine the plasma in an axial direction, wherein each of the respective
coaxial electrodes includes a rod-shaped center electrode that extends on
a single axis, a guide electrode that surrounds a facing front end
portion of the center electrode with a predetermined gap therebetween,
and an insulation member that insulates the center electrode and the
guide electrode from each other, wherein the insulation member is formed
of partially porous ceramics that include an insulative dense portion
which does not permit the continuous permeation of the liquefied plasma
medium and a porous portion which permits the continuous permeation of
the liquefied plasma medium, and wherein the insulative dense portion
includes a reservoir that holds the plasma medium therein, and by the
porous portion, the inner surface of the reservoir communicates with the
gap between the center electrode and the guide electrode, through the
inside of the insulative dense portion.

2. The plasma light source according to claim 1, further comprising: a
temperature-adjustable heating device that heats the insulation member so
as to liquefy the plasma medium therein.

3. The plasma light source according to claim 1, further comprising: a
gas-supply device that supplies an inert gas into the reservoir; and a
pressure-adjusting device that adjusts a supply pressure of the inert
gas.

4. A plasma light source comprising: a pair of coaxial electrodes that
face each other; a discharge-environment-maintaining device that
maintains a plasma medium inside the coaxial electrodes at a temperature
and a pressure appropriate for the generation of plasma; and a
voltage-applying device that applies discharge voltages having reversed
polarities to the respective coaxial electrodes, wherein a tubular
discharge is formed between the pair of coaxial electrodes so as to
confine the plasma in an axial direction, wherein each of the respective
coaxial electrodes includes a rod-shaped center electrode that extends on
a single axis, a guide electrode that surrounds a facing front end
portion of the center electrode with a predetermined gap therebetween,
and an insulation member that insulates the center electrode and the
guide electrode from each other, wherein the insulation member is formed
of porous ceramics that has a front surface positioned at the side of the
front end portion of the center electrode and has a rear surface at the
opposite side thereof, and wherein the plasma light source further
comprises: a hollow reservoir that is opened to the rear surface of the
insulation member and holds the plasma medium therein; a gas-supply
device that supplies an inert gas into the reservoir; a
pressure-adjusting device that adjusts a supply pressure of the inert
gas; and a temperature-adjustable heating device that heats and liquefies
the plasma medium inside the reservoir.

5. The plasma light source according to claim 1, wherein the
voltage-applying device includes a positive voltage source that applies a
positive discharge voltage which is higher than that of the guide
electrode of one of the coaxial electrodes, to the center electrode of
the one of the coaxial electrodes, a negative voltage source that applies
a negative discharge voltage which is lower than that of the guide
electrode of the other of the coaxial electrodes, to the center electrode
of the other of the coaxial electrodes, and a trigger switch that causes
the positive voltage source and the negative voltage source to
simultaneously apply the discharge voltages to the respective coaxial
electrodes.

6. The plasma light source according to claim 4, wherein the
voltage-applying device includes a positive voltage source that applies a
positive discharge voltage which is higher than that of the guide
electrode of one of the coaxial electrodes, to the center electrode of
the one of the coaxial electrodes, a negative voltage source that applies
a negative discharge voltage which is lower than that of the guide
electrode of the other of the coaxial electrodes, to the center electrode
of the other of the coaxial electrodes, and a trigger switch that causes
the positive voltage source and the negative voltage source to
simultaneously apply the discharge voltages to the respective coaxial
electrodes

7. The plasma light source according to claim 2, further comprising: a
gas-supply device that supplies an inert gas into the reservoir; and a
pressure-adjusting device that adjusts a supply pressure of the inert
gas.

Description:

[0002] Lithography which uses an extreme ultraviolet light source for the
microfabrication of next-generation semiconductors has been expected.
Lithography is a technique which reduces and projects light or beams onto
a silicon substrate through a mask having a circuit pattern drawn thereon
and which forms an electronic circuit by exposing a resist material. The
minimal processing dimensions of the circuit formed by optical
lithography are basically dependent on the wavelength of the light
source. Accordingly, the wavelength of the light source used for the
development of next-generation semiconductors needs to be shortened, and
thus a study for the development of such a light source has been
conducted.

[0003] Extreme ultraviolet (EUV) is most expected as the next-generation
lithography light source, and the light has a wavelength in the range of
approximately 1 to 100 nm. Since the light of the range has high
absorptivity with respect to all materials, and a transmissive optical
system such as a lens cannot be used, a reflective optical system is
used. Further, it is very difficult to develop the optical system of the
EUV light range, and this optical system exhibits reflection
characteristics only for a restricted wavelength.

[0004] Currently, a Mo/Si multilayer film reflection mirror with
sensitivity of 13.5 nm has been developed. Then, by developing
lithography techniques obtained by the combination of the light of this
wavelength and the reflection mirror, it is expected that processing
dimensions of 30 nm or less may be realized. In order to realize a new
microfabrication technique, there is an immediate need for the
development of a lithography light source with a wavelength of 13.5 nm,
and radiant light from plasma with high energy density has gained
attention.

[0005] The generation of light source plasma may be largely classified
into laser produced plasma (LPP) using the radiation of laser and
discharge produced plasma (DPP) using the discharge of a gas and driven
by the pulse power technique. In DPP, the input power is directly
converted into plasma energy. For this reason, the DPP has better energy
converting efficiency than that of the LPP, and has an advantage in that
the device is small and cheap.

[0006] The radiation spectrum from hot and highly dense plasma using the
DPP is basically determined by the temperature and the density of the
target material. According to the calculation result for the atomic
process of the plasma, in order to obtain plasma of the EUV radiation
range, the electron temperature and the electron density are respectively
optimized as about several 10 eV and 1018 cm-3 in the case of
Xe and Sn, and are respectively optimized as about 20 eV and 1018
cm-3 in the case of Li.

[0014] Regarding the EUV lithography light source, there is a demand for
the average output to be high, the size of the light source to be minute,
the amount of the scattering particles (debris) to be small, and the
like. At present, the EUV emitting amount is extremely low compared to
the output demand, and an increase in output is a major challenge to be
overcome. However, when the input energy is set to be large to obtain a
high output, damage caused by the thermal load reduces the life span of
the plasma generating device or the optical system. Accordingly, in order
to meet both high EUV output and low thermal load, high energy converting
efficiency is essentially needed.

[0015] At the beginning of forming plasma, a great deal of energy is
consumed for heating or ionization, and hot and highly dense plasma
radiating EUV is generally expanded rapidly. For this reason, the
radiation sustaining time T is extremely short. Accordingly, in order to
improve the converting efficiency, it is important to maintain the plasma
in a hot temperature and a highly dense state appropriate for EUV
radiation for a long period of time (in an order of μsec).

[0016] A solid medium such as Sn or Li at a room temperature has a high
spectrum converting efficiency, but a change in phase such as melting and
evaporation occurs with the generation of plasma. For this reason, the
inside of the device may be greatly contaminated by debris (derivatives
produced with the discharge) such as neutral particles. Therefore, there
is a similar demand for the reinforcement of a system which supplies and
collects the target.

[0017] Currently, the radiation time of the general EUV plasma light
source is about 100 nsec, so that the output is extremely insufficient.
In order to obtain both high converting efficiency and high average
output for the industrial application, there is a need to attain an EUV
radiation time of several μsec for one shot. That is, in order to
develop a plasma light source with high converting efficiency, there is a
need to maintain plasma in a temperature-and-density state appropriate
for the target for several μsec (at least 1 μsec or more) to attain
the stable EUV radiation.

[0018] In addition, in a capillary discharge of the related art, since the
plasma is confined inside the capillary, there is a disadvantage in that
the effective radiation solid angle is small.

[0019] Therefore, it is an object of the invention to provide a plasma
light source which stably generates plasma light for EUV radiation for a
long period of time (in an order of μsec), which suffers little damage
due to the thermal load of the component, and which obtains a large
effective radiation solid angle of the plasma light.

Means for Solving the Problems

[0020] In order to attain the above-described object, according to a first
invention, there is provided a plasma light source comprising:

[0021] a pair of coaxial electrodes that face each other;

[0022] a discharge-environment-maintaining device that maintains a plasma
medium inside the coaxial electrodes at a temperature and a pressure
appropriate for the generation of plasma; and

[0023] a voltage-applying device that applies discharge voltages having
reversed polarities to the respective coaxial electrodes,

[0024] wherein a tubular discharge is formed between the pair of coaxial
electrodes so as to confine the plasma in the axial direction,

[0025] wherein each of the respective coaxial electrodes includes a
rod-shaped center electrode that extends on a single axis, a guide
electrode that surrounds the facing front end portion of the center
electrode with a predetermined gap therebetween, and an insulation member
that insulates the center electrode and the guide electrode from each
other,

[0026] wherein the insulation member is formed of partially porous
ceramics that include an insulative dense portion which does not permit
the continuous permeation of the liquefied plasma medium and a porous
portion which permits the continuous permeation of the liquefied plasma
medium, and

[0027] wherein the insulative dense portion includes a reservoir that
holds the plasma medium therein, and by the porous portion, the inner
surface of the reservoir communicates with the gap between the center
electrode and the guide electrode, through the inside of the insulative
dense portion.

[0028] According to the preferred embodiments of the first invention, the
plasma light source further includes a temperature-adjustable heating
device that heats the insulation member so as to liquefy the plasma
medium therein.

[0029] According to the preferred embodiments of the first invention, the
plasma light source further includes a gas-supply device that supplies an
inert gas into the reservoir, and a pressure-adjusting device that
adjusts a supply pressure of the inert gas.

[0030] In order to attain the above-described object, according to a
second invention, there is provided a plasma light source comprising:

[0031] a pair of coaxial electrodes that face each other;

[0032] a discharge-environment-maintaining device that maintains a plasma
medium inside the coaxial electrodes at a temperature and a pressure
appropriate for the generation of plasma; and

[0033] a voltage-applying device that applies discharge voltages having
reversed polarities to the respective coaxial electrodes,

[0034] wherein a tubular discharge is formed between the pair of coaxial
electrodes so as to confine the plasma in the axial direction,

[0035] wherein each of the respective coaxial electrodes includes a
rod-shaped center electrode that extends on a single axis, a guide
electrode that surrounds the facing front end portion of the center
electrode with a predetermined gap therebetween, and an insulation member
that insulates the center electrode and the guide electrode from each
other,

[0036] wherein the insulation member is formed of porous ceramics that has
a front surface positioned at the side of the front end portion of the
center electrode and has a rear surface at the opposite side thereof, and

[0037] wherein the plasma light source further comprises:

[0038] a hollow reservoir that is opened to the rear surface of the
insulation member and holds the plasma medium therein;

[0039] a gas-supply device that supplies an inert gas into the reservoir;

[0040] a pressure-adjusting device that adjusts a supply pressure of the
inert gas; and

[0041] a temperature-adjustable heating device that heats and liquefies
the plasma medium inside the reservoir.

[0042] According to the preferred embodiments of the first or second
invention, the voltage-applying device may include a positive voltage
source that applies a positive discharge voltage which is higher than
that of the guide electrode of one of the coaxial electrodes, to the
center electrode of the one of the coaxial electrodes, a negative voltage
source that applies a negative discharge voltage which is lower than that
of the guide electrode of the other of the coaxial electrodes, to the
center electrode of the other of the coaxial electrodes, and a trigger
switch that causes the positive voltage source and the negative voltage
source to simultaneously apply the discharge voltages to the respective
coaxial electrodes.

Advantageous Effect of the Invention

[0043] According to the device of the first and second inventions, the
pair of facing coaxial electrodes are provided, the planar discharge
current (the planar discharge) is generated in each of the pair of
coaxial electrodes, a single plasma is formed at the middle position
between the respective facing coaxial electrodes by the planar
discharges, and then the planar discharges are connected to form the
tubular discharge between the pair of coaxial electrodes so as to
generate the magnetic field (the magnetic bottle) which confines the
plasma, thereby stably generating the plasma light for EUV radiation for
a long period of time (in an order of μsec).

[0044] Further, since the single plasma is formed at the middle position
between the pair of facing coaxial electrodes, and the energy converting
efficiency may be improved to a great extent compared to a capillary
discharge or a vacuum discharge metal plasma of the related art, the
thermal load of each electrode is reduced during the formation of plasma,
so that damage caused by the thermal load of the component may be
remarkably reduced.

[0045] Further, since the plasma which is the light emitting source of the
plasma light is formed at the middle position between the pair of facing
coaxial electrodes, the effective radiation solid angle of the generated
plasma light may be made large.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a diagram showing a first embodiment of a plasma light
source according to the invention.

[0054] Hereinafter, exemplary embodiments of the invention will be
described on the basis of the accompanying drawings. Furthermore, the
same reference numerals will be given to common parts in the respective
drawings, and repetitive description thereof will be omitted.

[0055]FIG. 1 is a diagram showing a first embodiment of a plasma light
source according to the invention.

[0056] In this drawing, a plasma light source according to a first
embodiment of the invention includes a pair of coaxial electrodes 10, a
discharge-environment-maintaining device 20, a voltage-applying device
30, and a heating device 40.

[0057] The pair of coaxial electrodes 10 are arranged so as to face each
other with respect to a symmetry plane 1.

[0058] Each coaxial electrode 10 includes a rod-shaped center electrode
12, a guide electrode 14, and an insulation member 16.

[0059] The rod-shaped center electrode 12 is a conductive electrode which
extends on a single axis Z-Z.

[0060] In this example, the end surface of the center electrode 12 facing
the symmetry plane 1 has a circular-arc shape. Furthermore, this
structure is not essential, and a recessed portion may be formed in the
end surface so as to stabilize a planar discharge current 2 and a tubular
discharge 4 to be described later, or the end surface may be a plane.

[0061] The guide electrode 14 surrounds the facing front end portion of
the center electrode 12 with a predetermined gap therebetween, and holds
a plasma medium therebetween. The guide electrode 14 includes, in this
example, a small diameter hollow cylindrical portion 14a which is
positioned at the side of the symmetry plane 1, and a large diameter
hollow portion 14b which is positioned at the opposite side thereof and
has a larger diameter than that of the small diameter hollow cylindrical
portion 14a. Further, the end surface of the small diameter hollow
cylindrical portion 14a facing the symmetry plane 1 in the guide
electrode 14 has a circular-arc shape in this example, but may have a
plane shape.

[0062] In this example, the plasma medium may be a solid plasma medium
such as Sn and Li at a room temperature.

[0063] The insulation member 16 is an electrical insulator which is
positioned between the center electrode 12 and the guide electrode 14,
and has a hollow cylindrical shape, and electrically insulates the center
electrode 12 and the guide electrode 14 from each other. The insulation
member 16 is formed of porous ceramics having a front surface positioned
at the side of the front end portion of the center electrode 12 and a
rear surface positioned at the opposite side thereof.

[0064] In this example, the insulation member 16 includes a small diameter
portion which is fitted into the inside of the small diameter hollow
cylindrical portion 14a, and a large diameter portion which is fitted
into the inside of the large diameter hollow portion 14b. The large
diameter portion is integrally connected to the guide electrode 14 by a
bolt 17 (refer to FIG. 2).

[0065] In the above-described pair of coaxial electrodes 10, the
respective center electrodes 12 are positioned on the same axis Z-Z, and
are positioned so as to be symmetry to each other with a predetermined
gap therebetween.

[0066] The discharge-environment-maintaining device 20 maintains the
coaxial electrode 10 to be in a state where the plasma medium inside the
coaxial electrode 10 has a temperature and a pressure appropriate for the
generation of plasma.

[0067] The discharge-environment-maintaining device 20 may include, for
example, a vacuum chamber, a temperature controller, a vacuum device, and
a plasma medium supply device. Furthermore, this structure is not
essential, and the other structures may be used.

[0070] The positive voltage source 32 applies a positive discharge
voltage, which is higher than that of the guide electrode 14 of one of
the coaxial electrodes 10 (in this example, the left electrode 10), to
the center electrode 12 of the same coaxial electrode 10.

[0071] The negative voltage source 34 applies a negative discharge
voltage, which is lower than that of the guide electrode 14 of the other
of the coaxial electrodes 10 (in this example, the right electrode 10),
to the center electrode 12 of the same coaxial electrode 10.

[0073] With this structure, the plasma light source according to the first
embodiment of the invention is configured to confine plasma in the axial
direction by forming a tubular discharge (described later) between the
pair of coaxial electrodes 10.

[0074] The heating device 40 includes an electric heater 42 which heats
the insulation member 16, and a heating power supply 44 which supplies
heating power to the electric heater 42. The heating device 40 heats the
insulation member 16 so as to liquefy the plasma medium therein.
Especially, the heating device 40 heats and liquefies the plasma medium
inside a reservoir 18 described later by heating the insulation member
16.

[0075] In this example, the electric heater 42 is arranged in a groove
formed in the outer periphery of the large diameter portion of the
insulation member 16. In this example, the electric heater 42 receives
power from the heating power supply 44 through a power supply line which
penetrates the large diameter hollow portion 14b of the guide electrode
14. Further, the electric heater 42 includes a temperature sensor (not
shown) to heat the insulation member 16 and maintain the insulation
member 16 at a predetermined temperature.

[0077] In this drawing, the insulation member 16 is formed of partially
porous ceramics obtained by integrally molding an insulative dense
portion and a porous portion such that the liquefied plasma medium does
not continuously permeate the insulative dense portion 16a, but
continuously permeates the porous portion 16b.

[0078] The insulative dense portion 16a insulates the center electrode 12
and the guide electrode 14 from each other.

[0079] Further, in this example, the porous portion 16b continuously
extends from the rear surface of the insulation member 16 to the front
surface thereof through the inside of the insulative dense portion 16a.

[0081] Further, the particle diameter and the burning temperature of the
insulative dense portion 16a are set so that the liquefied plasma medium
does not continuously permeate the insulative dense portion 16a.
Moreover, the particle diameter and the burning temperature of the porous
portion 16b are set so that the liquefied plasma medium continuously
permeate the insulative dense portion 16a.

[0082] Further, the insulative dense portion 16a includes the reservoir 18
which holds the plasma medium therein. In this example, the reservoir 18
is a cylindrical cavity which is provided inside the insulative dense
portion 16a and which centers the axis Z-Z.

[0083] Furthermore, in this example, the reservoir 18 is opened to the
rear surface of the insulation member 16, and the rear surface (the left
side of the drawing) of the reservoir 18 is closed by a closing plate 15.
The closing plate 15 is attachably and detachably fixed by the nut 13
into which a screw shaft 12a provided at the rear surface side of the
center electrode 12 is screwed. The closing plate 15 may be formed of a
heat-resistant metal plate or heat-resistant ceramics which withstands
the temperature of the liquefied plasma medium.

[0084] With this structure, the reservoir 18 may be appropriately
replenished with the plasma medium by the attachment and detachment of
the closing plate 15. Further, in this example, the plasma medium inside
the reservoir 18 is Sn, Li, or the like, and is liquefied by the heating
device 40.

[0085] The plasma light source of FIG. 1 further includes a gas-supply
device 50 and a pressure-adjusting device 52.

[0086] The gas-supply device 50 supplies an inert gas into the reservoir
18. It is desirable that the inert gas be a rare gas such as argon and
xenon.

[0087] The pressure-adjusting device 52 is installed at an intermediate
position in the gas supply line of the gas-supply device 50 so as to
adjust the supply pressure of the inert gas.

[0088] Furthermore, in the first embodiment, the gas-supply device 50 and
the pressure-adjusting device 52 may be omitted.

[0089] By using the above-described plasma light source, the insulation
member 16 is heated and maintained at a temperature at which the vapor
pressure of the plasma medium 6 (Sn, Li, and the like) becomes a pressure
(in an order of Torr) appropriate for the generation of plasma, and the
inside of the coaxial electrode 10 (the gap between the center electrode
12 and the guide electrode 14) is made to be a vapor atmosphere of the
plasma medium 6 having a pressure in an order of Torr.

[0090] Further, the electrode conductors (the center electrode 12 and the
guide electrode 14) are maintained at a high temperature at which the
vapor of the plasma medium 6 does not aggregate.

[0091] The plasma medium 6 may be made to flow out in a liquid metal state
from the surface (the end surface) of the porous portion 16b of the
insulation member 16 to the inside of the coaxial electrode 10 (the gap
between the center electrode 12 and the guide electrode 14).

[0092] Instead, the plasma medium 6 may be supplied as a metal vapor gas
from the surface (the end surface) of the porous portion 16b of the
insulation member 16 to the inside of the coaxial electrode 10 (the gap
between the center electrode 12 and the guide electrode 14). In this
case, the heating device 40 liquefies the plasma medium 6 inside the
reservoir 18, and vaporizes the liquefied plasma medium 6 so as to be
changed into a metal vapor gas. Furthermore, in order to supply the
plasma medium 6 as a metal vapor gas from the surface (the end surface)
of the porous portion 16b to the inside of the coaxial electrode 10, it
is desirable that the insulative dense portion 16b be formed such that a
gas cannot pass through the insulative dense portion 16b.

[0093] Furthermore, the shapes of the insulative dense portion 16a and the
porous portion 16b are not limited to this example, and may be other
shapes as long as the center electrode 12 and the guide electrode 14 are
electrically insulated from each other.

[0094] FIGS. 3A to 3D are diagrams illustrating the operation of the
plasma light source of FIG. 1. FIG. 3A shows the state where the planar
discharge is generated, FIG. 3B shows the state where the planar
discharge moves, FIG. 3C shows the state where the plasma is formed, and
FIG. 3D shows the state where the plasma-confining magnetic field is
formed.

[0095] Hereinafter, referring to these drawings, a method of generating
plasma light using the device of the first embodiment of the invention
will be described.

[0096] In the plasma light source according to the first embodiment of the
invention, the above-described pair of coaxial electrodes 10 are disposed
so as to face each other, the plasma medium is supplied to the insides of
the coaxial electrodes 10 by the discharge-environment-maintaining device
20 so as to be maintained at a temperature and a pressure appropriate for
the generation of plasma, and discharge voltages having reversed
polarities are applied to the respective coaxial electrodes 10 by the
voltage-applying device 30.

[0097] As shown in FIG. 3A, due to the application of voltages, a planar
discharge current (hereinafter, referred to as a planar discharge 2) is
generated in the surfaces of the insulation members 16 of the pair of
coaxial electrodes 10. The planar discharge 2 is a planar discharge
current which is spread two-dimensionally, and is referred to as "a
current sheet" hereinafter.

[0098] In addition, at this time, a positive voltage (+) is applied to the
center electrode 12 of the left coaxial electrode 10, a negative voltage
(-) is applied to the guide electrode 14, a negative voltage (-) is
applied to the center electrode 12 of the right coaxial electrode 10, and
a positive voltage (+) is applied to the guide electrode 14.

[0099] Furthermore, both guide electrodes 14 may be grounded so as to be
maintained at 0 V, a positive voltage (+) may be applied to one center
electrode 12, and a negative voltage (-) may be applied to the other
center electrode 12.

[0100] As shown in FIG. 3B, the planar discharge 2 moves in the direction
(the direction toward the center of the drawing) of being discharged from
the electrode by the self magnetic field.

[0101] As shown in FIG. 3C, when the planar discharge 2 reaches the front
ends of the pair of coaxial electrodes 10, the plasma medium 6 which is
interposed between the pair of planar discharges 2 is changed to have a
high density and a high temperature, so that a single plasma 3 is formed
at the middle position (the symmetry plane 1 of the center electrode 12)
between the facing coaxial electrodes 10.

[0102] Further, in this state, the pair of facing center electrodes 12 are
respectively held at a positive voltage (+) and a negative voltage (-),
and the pair of facing guide electrodes 14 are respectively held at a
positive voltage (+) and a negative voltage (-). Accordingly, as shown in
FIG. 3D, the planar discharges 2 of the pair of facing center electrodes
12 may be connected to each other to form the tubular discharge 4
discharged between the pair of facing guide electrodes 14. Here, the
tubular discharge 4 indicates a hollow cylindrical discharge current
which surrounds the axis Z-Z.

[0103] When the tubular discharge 4 is formed, a plasma-confining magnetic
field (a magnetic bottle) which is indicated by the reference numeral 5
is formed so as to confine the plasma 3 in the radial direction and the
axial direction.

[0104] That is, when the magnetic bottle 5 is formed in a shape of which
the center portion is large and both sides are small due to the pressure
of the plasma 3, an axial magnetic pressure gradient toward the plasma 3
is formed, and the plasma 3 is confined at the middle position by the
magnetic pressure gradient. Moreover, the plasma 3 is compressed
(Z-pinched) toward the center by the self magnetic field of the plasma
current, so that it is also confined in the radial direction by the self
magnetic field.

[0105] In this state, the energy corresponding to the light emitting
energy of the plasma 3 is continuously supplied from the voltage-applying
device 30, so that it is possible to stably generate the plasma light 8
(EUV) with a high energy converting efficiency for a long period of time.

[0106]FIG. 4 is a diagram showing a second embodiment of a plasma light
source according to the invention, and FIG. 5 is an enlarged view of the
coaxial electrode of FIG. 4.

[0107] In the second embodiment, a center electrode 12B, a guide electrode
14B, an insulation member 16B, a reservoir 18B, and an electric heater
42B are respectively provided instead of the center electrode 12, the
guide electrode 14, the insulation member 16, the reservoir 18, and the
electric heater 42 of the first embodiment.

[0108] In FIGS. 4 and 5, each coaxial electrode 10 includes a rod-shaped
center electrode 12B, a tubular guide electrode 14B, and a ring-shaped
insulation member 16B.

[0109] The ring-shaped insulation member 16B is an electrical insulator
which is positioned between the center electrode 12B and the guide
electrode 14B, and has a hollow cylindrical shape, and electrically
insulates the center electrode 12B and the guide electrode 14B from each
other. In this example, the ring-shaped insulation member 16B is formed
of porous ceramics.

[0110] Further, the plasma light source of FIG. 4 further includes a
hollow reservoir 18B which is opened to the rear surface of the
insulation member 16B and which holds the plasma medium therein.

[0111] Moreover, the heating device 40 includes, in this example, an
electric heater 42B which heats the reservoir 18B, and a heating power
supply 44 which supplies heating power to the electric heater 42B. The
heating device 40 heats and liquefies the plasma medium inside the
reservoir 18B.

[0112] The other structures of the second embodiment are the same as those
of the first embodiment. However, in the second embodiment, the
gas-supply device 50 and the pressure-adjusting device 52 are not
omitted.

[0113] According to the device of the above-described first or second
embodiment of the invention, the pair of facing coaxial electrodes 10 are
provided, the planar discharge current (the planar discharge 2) is
generated in each of the pair of coaxial electrodes 10, the single plasma
3 is formed at the middle position between the respective facing coaxial
electrodes 10 by the planar discharges 2, and then the planar discharges
2 are connected to form the tubular discharge 4 between the pair of
coaxial electrodes so as to generate the plasma-confining magnetic field
5 (the magnetic bottle 5) which confines the plasma 3, thereby stably
generating the plasma light for EUV radiation for a long period of time
(in an order of μsec).

[0114] Further, since the single plasma 3 is formed at the middle position
between the pair of facing coaxial electrodes 10, and the energy
converting efficiency is improved to a great extent (ten times or more)
compared to a capillary discharge or a vacuum discharge metal plasma of
the related art, the thermal load of each electrode is reduced during the
formation of plasma, so that damage caused by the thermal load of the
component may be remarkably reduced.

[0115] Further, since the plasma 3 which is the light emitting source of
the plasma light is formed at the middle position between the pair of
facing coaxial electrodes 10, the effective radiation solid angle of the
plasma light can be made large.

[0116] Moreover, in the first embodiment of the invention, the insulation
member 16 is formed of the partially porous ceramics obtained by
integrally molding the insulative dense portion 16a and the porous
portion 16b, the insulative dense portion 16a is provided with the
reservoir 18 which holds the plasma medium therein, and by the porous
portion 16b, the inner surface of the reservoir 18 communicates with the
gap between the center electrode 12 and the guide electrode 14, through
the inside of the insulative dense portion 16a. Accordingly, due to the
presence of the insulative dense portion 16a, the insulation in the
coaxial electrode may be maintained even when a liquid metal as a plasma
medium flows through the porous portion 16b, whereby the plasma medium
may be continuously supplied between the center electrode 12 and the
guide electrode 14.

[0117] Furthermore, it is desirable that the insulation member 16 be
obtained by integrally molding the insulative dense portion 16a and the
porous portion 16b in consideration of the structure of the device.
However, the insulative dense portion 16a and the porous portion 16b may
be bonded to each other (by adhering, brazing, or the like), or a seal
structure is provided to prevent the plasma medium from leaking from a
clearance between the insulative dense portion 16a and the porous portion
16b.

[0118] Moreover, in the first embodiment or the second embodiment of the
invention, there are provided the hollow reservoir 18 or 18B which is
opened to the rear surface of the insulation member 16 or 16B and which
holds the plasma medium therein, the gas-supply device 50 which supplies
the inert gas into the reservoir 18 or 18B, the pressure-adjusting device
52 which adjusts the supply pressure of the inert gas, and the
temperature-adjustable heating device 40 which heats and liquefies the
plasma medium inside the reservoir 18 or 18B. Accordingly, it is possible
to control the vapor pressure of the plasma medium at the front surface
of the insulation member 16 or 16B through the temperature adjustment of
the heating device 40. Further, at the same time, it is possible to
control the supply amount of the plasma medium (the liquid metal) by
adjusting the pressure of the inert gas to be supplied into the reservoir
18 or 18B by the gas-supply device 50 and the pressure-adjusting device
52.

[0119] Therefore, it is possible to continuously supply the plasma medium,
supply the plasma medium at the sufficient supply speed, and
independently control the supply amount and the vapor pressure of the
plasma medium.

[0120] Furthermore, it should be understood that the invention is not
limited to the above-described embodiments and all modifications may be
included in the scope of the appended claims or the equivalents thereof.